Arabidopsis ATP A2 peroxidase. Expression and high-resolution structure ofa plant peroxidase with implications for lignification

Lignins are phenolic biopolymers synthesized by terrestrial, vascular plant s for mechanical support and in response to pathogen attack. Peroxidases ha ve been proposed to catalyse the dehydrogenative polymerization of monolign ols into lignins, although no specific isoenzyme has been shown to be invol ved in lignin biosynthesis. Recently we isolated an extracellular anionic p eroxidase, ATP A2, from rapidly lignifying Arabidopsis cell suspension cult ure and cloned its cDNA. Here we show that the Atp A2 promoter directs GUS reporter gene expression in lignified tissues of transgenic plants. Moreove r, an Arabidopsis mutant with increased lignin levels compared to wild type shows increased levels of ATP A2 mRNA and of a mRNA encoding an enzyme ups tream in the lignin biosynthetic pathway. The substrate specificity of ATP A2 was analysed by X-ray crystallography and docking of lignin precursors. The structure of ATP A2 was solved to 1.45 Angstrom resolution at 100 K. Do cking of p-coumaryl, coniferyl and sinapyl alcohol in the substrate binding site of ATP A2 were analysed on the basis of the crystal structure of a ho rseradish peroxidase C-CN-ferulic acid complex. The analysis indicates that the precursors p-coumaryl and coniferyl alcohols are preferred by ATP A2, while the oxidation of sinapyl alcohol will be sterically hindered in ATP A 2 as well as in all other plant peroxidases due to an overlap with the cons erved Pro-139. We suggest ATP A2 is involved in a complex regulation of the covalent cross-linking in the plant cell wall.